In a semiconductor integrated circuit device, a silicon nitride film is widely utilized as an insulating film, a protective film, or the like. As a forming method of the silicon nitride film, there is known a plasma CVD method using a silicon source gas such as monosilane (SiH4) or disilane (Si2H6) and a nitrogen-containing gas such as a nitrogen gas or an ammonia gas.
Meanwhile, it is also known that the silicon nitride film formed by the plasma CVD contains hydrogens (see, for example, Non-Patent Document 1) therein. Due to the presence of the hydrogens in the silicon nitride film, the silicon nitride film has a film characteristic worse than that of a silicon nitride film formed by heat. To solve this problem, Patent Document 1 describes a technique of reducing an amount of the hydrogen contained in the silicon nitride film by forming the silicon nitride film through plasma CVD in which plasma excited by microwave is used and a processing temperature, a composition ratio of a processing gas, a microwave power and a processing pressure are regulated to certain ranges.
Patent Document 1: Japanese Patent Laid-open Publication No. 2009-246129
Non-Patent Document 1: Fuji Electric Journal Vol. 78 No. 4 (2005), pages 64 to 67
Recently, when forming the silicon nitride film by the plasma CVD, it is required to perform the processing at a lower temperature in order to meet requirements for high integration and high performance of LSI. If, however, the film formation is performed at a low temperature, the amount of the hydrogen contained in the silicon nitride film cannot be reduced sufficiently just by adjusting the film forming conditions, so that a required characteristic may not be achieved. For example, though a silicon nitride film to be used as a hard mask needs to be formed at a low temperature, if the silicon nitride film is formed at the low temperature, it is difficult to obtain high resistance (etching selectivity) in wet etching and dry etching, which is necessary for the hard mask.